1. Field of the Invention
The present invention relates to a particulate matter detection device, and more particularly, it relates to a particulate matter detection device having a high measurement sensitivity and a high measurement accuracy.
2. Description of the Related Art
A flue exhaust gas or a diesel engine exhaust gas includes a particulate matter (PM) such as soot, which has been a cause for air pollution. For the purpose of removing the particulate matter, a filter (a diesel particulate filter: DPF) made of a ceramic material or the like has widely been used. The DPF made of the ceramic material can be used for a long period of time, but defects such as cracks or melting damages due to thermal deterioration or the like might be generated, and a micro amount of the particulate matter might leak. When such defects are generated, from the viewpoint of the prevention of the air pollution, it is remarkably important to immediately detect the generation of the defects, thereby recognizing the abnormality of a device.
As a method of detecting the generation of such defects, there is disclosed a method of disposing a particulate matter detection device on a downstream side of the DPF (e.g., see Patent Documents 1 and 2).
For example, the particulate matter detection device disclosed in Patent Document 1 includes a detection device main body which includes a through hole formed in one end thereof and which is long in one direction, and at least a pair of electrodes embedded in a wall which forms this through hole and covered with a dielectric material. It is possible to electrically adsorb, by the wall surface of this through hole, a charged particulate matter included in a fluid flowing into the through hole, or a particulate matter charged by discharge which occurs in the through hole when a voltage is applied across the pair of electrodes and included in the fluid flowing into the through hole. When a change of electric characteristics of the wall which forms the through hole is measured, it is possible to detect a mass of the particulate matter adsorbed by the wall surface of the through hole.
Consequently, the conventional particulate matter detection device allows the particulate matter included in a measurement target gas to adhere to and around the pair of electrodes which are sensors, and measures the change of, the electric characteristics between the pair of electrodes, to detect the particulate matter in the measurement target gas.
Moreover, for example, as shown in FIG. 5 of Patent Document 2, as the pair of electrodes which are sensors for measurement, there are suggested a pair of electrodes which are branched into a plurality of electrodes, respectively, so that the branched electrodes face each other and a plurality of facing portions are present. When such electrodes are used and, for example, an electrostatic capacity is measured as electric characteristics between the pair of electrodes, a measurement sensitivity of the electrodes can be enhanced.